December 12 is the anniversary of The Bombing of Piazza Fontana in 1969, killing 17 people, and wounding another 88. Although the bombing was initially pinned on Anarchists, eventually it was discovered that the fascist group Ordine Nuovo was behind the bombings. A group with links to the US.
Now, I wish to preface this by saying that US involvement in this particular incident has not yet been proven. However, it is known that Operation Gladio was a very real program ran by the CIA. During the time period in question, Italy had a powerful communist party, and the US was quite eager to prevent them from being elected. According to General Gerardo Serraville, who commanded Gladio for some time, Gladio's role during the 1970s, was to "Fill the streets, creating a situation of such tension as to require military intervention", speaking about terrorist attacks that he perpetrated during the 70s and 80s in order to generate fear of the communist party. A 2000 report from the Italian government also claimed that the CIA was at minimum aware of the attack during its planning, and failed to report or act, but also implicates them in funding various fascist terrorist groups in Italy. The BBC has a great documentary on the subject as a whole.
There's also the matter of how quickly the crime scene was cleaned up, and evidence was destroyed. The anarchist they arrested first, "accidentally" died in a fall from the fourth floor of a police station. Over the years, repeated attempts have been made to bring the truth to light. We get glimpses now and then, such as with the Gladio reveal, and some US officers have gone to trial. As I mentioned at the outset, we still have no hard proof. But it would be in keeping with state sponsored terrorism that we do know about from that time period, continuing through the 80s and beyond.